1. Field of the Invention
The present invention relates to a process for producing bisphenol A exhibiting a stable hue. The thus produced bisphenol A is useful as raw materials of polycarbonate resins, epoxy resins, polyarylate resins or the like.
2. Background Arts
As well known in the arts, bisphenol A [2,2-bis(4-hydroxyphenyl) propane] is an important compound useful as raw material of engineering plastics such as polycarbonate resins and polyarylate resins, or epoxy resins. Recently, the demand for the above compound tends to be more and more increased. In particular, when used as raw material of polycarbonate resins, the bisphenol A has been required to exhibit a stable hue without undesired coloration even when it is treated at an elevated temperature.
It is also known that the bisphenol A is produced by condensing phenol with acetone in the presence of an acid-type ion exchange resin as a catalyst. In this case, the acid-type ion exchange resin is usually swelled with water and packed in a reactor. After packing, the ion exchange resin is washed with water to remove acid substances therefrom, and then washed with phenol prior to its use in the reaction. Conventionally, the phenol used for the washing has been usually mixed with a reaction mixture discharged from outlet of the reactor in order to recover and purify the phenol. For this reason, the thus produced bisphenol A inevitably contains acid substances, resulting in deteriorated hue thereof.
The present invention has been made in view of the above problems. An object of the present invention is to provide a process for producing bisphenol A exhibiting a stable hue without undesired coloration even when treated at an elevated temperature.
As the result of extensive studies, the present inventors have found that the above object is achieved by removing acid substances from a phenol solution obtained after washing the acid-type ion exchange resin. The present invention has been accomplished based on this finding.
Thus, the present invention provides a process for producing bisphenol A by reacting acetone with phenol in the presence of an acid-type ion exchange resin as a catalyst and then subjecting the reaction mixture to a reduced-pressure distillation to recover the bisphenol A from a fraction discharged from a bottom of distillation column, comprising:
washing said acid-type ion exchange resin filled in a reactor with phenol before using the ion exchange resin in the reaction; and
distilling a phenol solution obtained after the washing together with a fraction obtained from a top of the distillation column for recovery of phenol.
The present invention will be described in detail below.
First, the respective steps of the process for the production of bisphenol A is described.
Step (1): Reaction Step
Bisphenol A is produced by reacting acetone with an excess amount of phenol in the presence of an acid-type ion exchange resin as catalyst and, if required, alkylmercaptan as co-catalyst. As the suitable acid-type ion exchange resin as catalyst, there may be generally used sulfonic acid-type cation exchange resins. Examples of such sulfonic acid-type cation exchange resins include sulfonated styrene divinyl benzene copolymers, sulfonated cross-linked styrene polymers, phenol formaldehyde-sulfonic acid resins, benzene formaldehyde-sulfonic acid resins or the like. These sulfonic acid-type cation exchange resins may be used alone or in the form of a mixture of any two or more thereof.
The suitable alkylmercaptan used as co-catalyst are such mercaptans having a C1-C10 alkyl group. Examples of the alkylmercaptans include methylmercaptan, ethylmercaptan, propylmercaptan, octylmercaptan, cyclohexylmercaptan or the like. Among these alkylmercaptans, ethylmercaptan is especially preferred. Meanwhile, these alkylmercaptans may be used alone or in the form of a mixture of any two or more thereof.
The above reaction may be desirably conducted by a fixed-bed continuous or batch method, though not limited thereto. When the reaction is conducted by fixed-bed continuous method, the liquid hourly space velocity (LHSV) used therein is usually in the range of 0.2 to 30 Hrxe2x88x921, preferably 0.5 to 6 Hrxe2x88x921.
As to the other reaction conditions, the reaction temperature is 60 to 100xc2x0 C.; the molar ratio of phenol to acetone is 6 to 13; and the molar ratio of acetone to mercaptan is 13 to 25.
The resultant reaction mixture contains, in addition to bisphenol A, unreacted phenol, unreacted acetone, catalysts, by-produced water, alkylmercaptan, and other by-products such as organic sulfur compounds and colored substances.
Step (2): Recovery of By-Produced Water and Unreacted Raw Materials
Then, the reaction mixture obtained in the step (1) is distilled under reduced pressure to remove unreacted acetone, by-produced water, alkylmercaptan, a part of unreacted phenol and the like from a top of distillation column and obtain a liquid mixture containing bisphenol A, phenol, etc., from the bottom thereof. The reduced-pressure distillation may be conducted at a temperature of 70 to 180xc2x0 C. under a pressure of 6.7 to 80.0 kPa. Upon such a distillation, the unreacted phenol is subjected to azeotropy, and removed out of the reaction system from the top of the distillation column.
Step (3): Concentration of Bisphenol A
The bottoms obtained by removing the above substances from the reaction mixture, is then distilled under reduced pressure to remove unreacted phenol therefrom and concentrate bisphenol A contained therein. The thus obtained concentrated residual solution is used as a raw material of the subsequent crystallization step. The concentration conditions are not particularly restricted, but the concentration process may be usually conducted at a temperature of 100 to 170xc2x0 C. under a pressure of 5.3 to 66.7 kPa. When the temperature is less than 100xc2x0 C., it is necessary to keep the reaction system under high vacuum condition. On the contrary, when the temperature is more than 170xc2x0 C., an additional heat-removal step is required upon conducting the subsequent crystallization step. The concentration of bisphenol A contained in the concentrated residual solution is in the range of 20 to 50% by weight, preferably 20 to 40% by weight. When the concentration of bisphenol A contained in the concentrated residual solution is less than 20% by weight, the recovery percentage of bisphenol A becomes lowered. On the contrary, when the concentration of bisphenol A in the residual solution is more than 50% by weight, it is difficult to transport a slurry obtained after the crystallization step.
Step (4): Crystallization
The concentrated residual solution obtained in the step (3) is cooled to a temperature of 40 to 70xc2x0 C. to crystallize an adduct of bisphenol A and phenol (hereinafter referred to merely as xe2x80x9cphenol adductxe2x80x9d), thereby obtaining a slurry. The cooling is conducted due to heat removal caused by evaporating water added to external heat exchanger and crystallizer. Next, the slurry-like concentrated residual solution is subjected to filtration, centrifugal separation, etc., and separated into the phenol adduct and a crystallization mother liquor containing by-products. The thus obtained crystallization mother liquor may be directly or partially recycled to the reactor, or recovered in the form of phenol and isopropenyl phenol by subjecting whole or part thereof to alkali decomposition. Alternatively, a part or whole of the crystallization mother liquor may be isomerized and recycled as raw material to the crystallization step (refer to Japanese Patent Laid-open No. 6-321834).
Step (5): Heat-Melting of Phenol Adduct
The 1:1 adduct of bisphenol A and phenol obtained in the step (4) in the form of crystals, is heat-melted at a temperature of 100 to 160xc2x0 C. to obtain a liquid mixture.
Step (6): Recovery of Bisphenol A
The liquid mixture obtained in the step (5) is distilled under reduced pressure to remove phenol and recover bisphenol A therefrom. The reduced-pressure distillation is conducted at a temperature of 150 to 190xc2x0 C. under a pressure of 1.3 to 13.3 kPa. In addition, the removal of residual phenol may be conducted by known methods such as steam-stripping.
Step (7): Granulation of Bispherol A
The molten bisphenol A obtained in the step (6) is formed into droplets using a granulating apparatus such as spray dryer, and cooled and solidified to obtain an aimed product. The droplets are produced by spraying, spreading or the like method, and cooled with nitrogen, air or the like.
In the present invention, the acid-type ion exchange resin filled in the reactor is washed with phenol before used in the reaction, and a phenol solution obtained after the washing (hereinafter referred to merely as a xe2x80x9cwashing phenol solutionxe2x80x9d) is distilled together with a fraction (containing acetone, by-produced water, phenol and alkylmercaptan) obtained from a top of the reduced-pressure distillation column used in the above step (2) to recover phenol therefrom and recycle the recovered phenol. In the distillation, only one distillation column may be used to separate the mixture into respective components by varying the distillation conditions. Alternatively, several distillation columns may be used to separate the mixture into respective components. The washing phenol solution and the fraction obtained from the top of the above reduced-pressure distillation column may be continuously or intermittently mixed together. In any case of the continuous and intermittent mixing methods, the distillation may be conducted in such a manner that the ratio of phenol contained in the washing phenol solution to that recovered from the fraction obtained from the top of the reduced-pressure distillation column is within the range of 1:0.01 to 1:2.
The thus recovered phenol contains no acid substances and is therefore reusable as the raw material.
Meanwhile, the washing phenol solution may be temporarily stored before use.
Next, the process of the present invention will be described in more detail below. However, these examples are only illustrative and not intended to limit the present invention thereto.